Paper backings are conventionally used in many pressure sensitive adhesive tapes. The paper may be impregnated with a material which is capable of binding the paper fibers together to increase the tensile strength of the paper, thereby reducing the tendency of the tape to split or delaminate. A pressure sensitive adhesive is typically applied to one side of the paper sheet while a release coating is typically applied to the opposite side. A barrier layer may be applied to one side of the paper as a base for the release coating, and a primer may optionally be applied to the other side of the paper to help adhere the adhesive. The coated sheet is then wound and cut into rolls to form an adhesive tape.
In many applications, it is important that the tape be extensible and conformable with minimum tape stiffness. For example, a tape used to mask curved and irregularly shaped surfaces preferably conforms closely to such surfaces without tearing. In addition to being extensible and conformable, it is preferable that such tapes have relatively smooth surfaces so as to minimize the amount of adhesive and release coating required to cover the surfaces, and to minimize seepage (for example, of paint) under the edge of the masking tape. It is also desirable that the tape be as thin as possible to provide an improved paint edge.
Extensibility and conformability in tapes have traditionally been achieved by mechanically creping the paper used in the tape backing using a creping blade to impart ridges to the paper web either before or after saturant is applied. See, for example, U.S. Pat. No. 2,214,006 (Ziegler). The degree of extensibility can be further controlled by adding a rubber-resin saturant to bind the fibers of the creped backing and varying the degree of vulcanization. See, for example, U.S. Pat. No. 2,410,078 (Kellgren).
The creping process and the use of a creped backing have several disadvantages associated therewith. One disadvantage is that the creping process is difficult to control due to wearing of the creping blade and variations in the angle of blade used. This presents problems in maintaining product uniformity and productivity. Additionally, the degree and uniformity of the crepe is highly dependent on the moisture content of the paper web, which may be difficult to control.
The creped, saturated backing also requires more adhesive and barrier or release coating to fill in the valleys resulting from the creping process. In addition, the creped backing imparts a crepe image on the adhesive surface when the tape is provided in roll form, reducing the adhesive effectiveness. Finally, the creped backing may cause irritation to the applicator's fingers with prolonged use.
In contrast, a smooth backing requires less adhesive coating. In addition, the adhesive can be coated more uniformly as a continuous coating onto a smooth backing. This results in improved adhesion properties when compared to comparable tapes comprising creped backing. For masking applications, a smooth backing gives an improved paint edge because the adhesive can be coated more uniformly onto a smoother surface, and the adhesive surface becomes less disrupted in roll form.
To overcome the problems associated with a creped backing, several attempts have been made to either smooth out the creped surface with additional mechanical means, or to increase the frequency of crepe lines to impart a "fine" crepe or microcrepe texture that still gives sufficient extensibility and conformability for masking applications.
U.S. Pat. No. 2,941,661 (Picard) describes an impregnated and unified creped paper backing having a superfine crepe structure. The creping procedure comprises the steps of subjecting a freshly formed paper web to superfine creping on a steel wet press roll using a steel creping blade which imparts at least about 40 crepe lines per lineal inch (15.7 lines per lineal cm) and calendering to reduce thickness and impart a higher degree of smoothness. This compares to a conventional creped backing used in masking tapes which has about 15 to 30 crepe lines per lineal inch (5.9 to 11.8 lines per lineal cm). Picard further discloses a fine creped paper having closely spaced rounded crepe ridges on one side and a number of very small cup and pocket-like indentations on the other. U.S. Pat. No. 4,849,278 (Stokes) describes a flexible paper web particularly useful as label stock for flexible, squeezable containers made by applying a fine crepe pattern to a paper substrate, saturating the substrate with a soft polymer saturant at about 15 to 75 parts by weight of saturant per 100 parts of fiber weight, and calendering the web to improve the surface smoothness to improve printability. Although the methods of Picard and Stokes both result in paper backings that are significantly smoother than conventional creped backings, the processes still rely on a creping blade to achieve the fine creped pattern. Thus, many of the difficulties associated with such processes (for example, blade wear, critical moisture content of the web, and non-uniform crepe surfaces) remain.
Several attempts have been made to manufacture an extensible paper backing that does not rely on a creping blade. Generally, a compressive force is applied to the paper web in a direction parallel to the face surfaces of the web while simultaneously applying pressure perpendicular to said face surfaces. The pressure is applied by passing the web between rollers operating at different speeds. The compression in the machine direction results in a substantial decrease in the length of the paper, while the pressure perpendicular to the web surface maintains high sheet density. Although this process does not employ a creping blade and papers made by this process are often referred to as "uncreped", a mechanical means is still employed to impart fine, closely spaced ridges in the paper backing to give extensibility and conformability. As a result, these backings are "creped" for purposes of the present invention. As with the creping process, the moisture content of the web is still critical to maintain product uniformity. Mechanisms employed for obtaining the compressed paper web are described in several prior patents.
U.S. Pat. No. 2,624,245 (Cluett), for example, describes a process for the manufacture of an "uncreped" extensible paper whereby a sufficiently moist paper web is subjected to compression in a direction parallel to the surfaces of the web, while the paper is under considerable pressure perpendicular to its surface to restrain it from creping. During this process, the fibers of the web are distorted and crowded together, without thickening of the web. Other mechanical techniques for producing an extensible paper are described in U.S. Pat. No. 3,011,545 (Welsh); U.S. Pat. No. 3,329,556 (McFalls); U.S. Pat. No. 3,359,156 (Freuler); and U.S. Pat. No. 3,630,837 (Freuler); U.S. Pat. No. 3,104,197 (Back); U.S. Pat. No. 4,637,859 (Trokhan); U.S. Pat. No. 3,503,495 (Gustafson); U.S. Pat. No. 4,551,199 (Weldon); U.S. Pat. No. 4,834,838 (Klowak); and U.S. Pat. No. 5,209,973 (Romanus Wille).
Flatback or smooth papers are commercially available. A flatback paper is one in which, once the fiber mat has been formed, it is not mechanically disturbed to generate extensibility or conformability. Flatback papers, however, are not typically employed in applications where extensibility and conformability are required as they typically exhibit elongation at break values of from about 2 to about 3%.
Various attempts have been made to increase the extensibility of flatback papers. For example, vinylcontaining fibers (e.g., polyvinyl alcohol or polyvinyl chloride fibers) have been incorporated into the fiber mat. It is believed that the vinyl fibers are bonded to the cellulosic fibers in the mat by passing the mat through a heated nip. Even with the addition of the vinyl fibers, however, the extensibility of the paper is only minimally increased.
A commercially available flatback tape that incorporates vinyl fibers in the paper backing, Scotch Brand Masking Tape for Sealing Applications, is available from Sumitomo 3M. The backing is saturated with styrene-butadiene/acrylate saturant. This tape is said to have an elongation at break of 7%, which is not considered to provide adequate extensibilty for masking intricate configurations. The tape is marketed for linear masking tape applications.
Other flatback tapes are known that have an elongation at break of 7-8%. These tapes also utilize a styrene-butadiene acrylate saturant. Again, however, these tapes are not considered to provide adequate extensibility for masking intricate configurations.
Thus, a need still exists for an adhesive tape that has a smooth, uncreped flatback paper backing having a high degree of extensibility and conformability where no mechanical means has been used to generate these properties in the backing. Eliminating the mechanical means for generating extensibility and conformability would simplify the production and reduce the cost of extensible backings, and would eliminate the variables of blade wear, critical moisture content, compressible roll composition, and roller speed differential associated with creped or otherwise mechanically roughened backings. Furthermore, a flatback backing requires less adhesive, barrier and release coating and provides an improved paint edge as well as improved adhesive properties.